The present invention relates to a control device for an alternator and, particularly, to a control device for a vehicle-mounted alternator for temporarily switching an output connection of the alternator from a storage battery to a high voltage electric load of the vehicle, the load being driven by a higher voltage than an ordinary output voltage of the alternator to be supplied to the battery to charge the battery, so that enough power is supplied to the load.
In order to melt frozen water on a front glass of a vehicle in a low temperature area within a short time, a system has been proposed in which an output voltage of an alternator is switched from a usual 14 V for charging a vehicle-mounted storage battery to a high voltage of 40-70 V temporarily to supply power of about 1 KW to a high voltage load such as a heater embedded in the front glass.
When a separately-excited alternator, that is, an alternator whose exciting current is supplied from a battery, is used as the vehicle-mounted alternator, the battery may be discharged during much of this high voltage driving. Therefore, in order to avoid such undesirable discharge of the battery, a self-excited alternator, that is, an alternator whose exciting current is supplied from an output thereof, is used. In such a case, however, another problem may occur. That is, a field coil of the self-excited alternator may be burnt by large field current due to high voltage applied thereacross.
U.S. Pat. No. 4,346,337 and Japanese Kokai (Patent) No. 63-302735 each disclose a field current limitation of a self-excited alternator. In both of these systems, an oscillator is provided to produce a fixed pulse waveform signal by which a field current control element is on-off controlled when a detection of field current exceeds a predetermined value. Since, in these systems, a voltage applied across the field coil is fixed to a certain value, for example, 40 V, the current limit value itself varies with variation of the output voltage of the alternator. Since the field current limit value at, for example, 80 V may be twice the limit value at 40 V, it is impossible to prevent the burning of the field coil completely.